1. Field of the Invention
The present invention relates to an aluminum weldment, i.e., a welded assembly of aluminum workpieces, and a method of welding such aluminum workpieces, specifically, a lap resistance welding method.
2. Description of the Prior Art
Resistance welding is a welding process in which electrodes of copper or the like are held in contact with workpieces and supply an electric current to regions of the workpieces that are to be welded while the workpieces are being pressed against each other, for raising the temperature of the workpiece regions due to resistive heating thereby to weld the workpiece regions.
Workpieces of aluminum produce a small amount of heat and diffuse generated heat to a large extent because they have a small specific resistance and a large thermal conductivity. To resistance-weld aluminum workpieces, it is necessary to supply a large welding current to the aluminum workpieces in order to generate an increased amount of heat. However, a large welding current causes the surfaces of the aluminum workpieces which contact the electrodes to produce a large amount of heat, which tends to form an alloy of copper and aluminum at the tip ends of the electrodes. The electrodes that are alloyed with aluminum have a reduced service life. Another problem is that the initial and running costs of the welding machine used are high as the welding machine needs to be large in capacity due to the large current requirement. The mechanical strength of the joined spot (hereinafter referred to as a "nugget") of the aluminum workpieces is liable to be lower than that of the base material. Particularly in spot welding, the mechanical strength of the nugget often drops to or below 60% of that of the base material. For this reason, an increased welding current is supplied to increase the joining area of aluminum workpieces, or an increased number of spots are produced to weld aluminum workpieces. However, increasing the welding current requires large-size and high-performance equipment, and increasing the number of spots results in an increased number of welding steps.
To solve the above problems, there has been proposed a resistance welding process which employs an insert of low electric conductivity that is interposed between joining surfaces of aluminum workpieces for a lower welding current requirement and a higher heating efficiency. One known insert comprises a thin film of zinc as disclosed in Japanese patent publication No. 54-41550. The thin film of zinc is interposed between regions of aluminum plates which are to be resistance-welded, and then instantaneously melted in a resistance welding process to produce a high-resistance area to concentrate electric energy thereon for thereby forming and separating a solid solution of aluminum and zinc in the nugget, which serves to increase the mechanical strength of the joined area. Japanese patent publication No. 59-26392 discloses an insert comprising a thin film of titanium to be interposed between aluminum workpieces to be welded, and Japanese patent publication No. 59-26393 discloses an insert comprising a thin film of stainless steel to be interposed between aluminum workpieces to be welded. It has also been proposed to employ an insert made of an alloy composed of 0.05 to 2.0 wt % of iron, 0.5 to 2.0 wt % of manganese, 0.7 wt % or less of magnesium, and a remainder of aluminum as disclosed in Japanese patent publication No. 63-278679.
However, none of the prior resistance welding processes have proven satisfactory as they have failed to meet demands for lower welding currents. Nuggets produced by the conventional resistance welding processes do not have a sufficient breaking strength, so that the joined regions may easily be peeled off when subjected to large external forces. For example, inserts in the form of thin films of zinc, titanium, and stainless steel fail to increase the mechanical strength of the nugget up to the mechanical strength of the base material, though they can increase the resistance locally. This is because the mechanical strength of a solid solution of zinc and aluminum is small as the difference between their lattice constants is small, and also because the effect of aluminum--titanium and aluminum--iron solid solutions is small due to very small limit amounts of titanium and iron, i.e., 0.15 wt % and 0.05 wt %, respectively, that are allowed to be added to form a solid solution with aluminum. In addition, an insert of zinc is disadvantageous in that the welded area has poor corrosion resistance. When an insert of titanium is used, the nugget is apt to become detective because some amount of titanium remains as an inclusion in the nugget.